The invention describes a device used to reduce fluid pressure in piping systems found in typical process plants. Pressure reduction of gases or liquids can reach from a few atmospheres to several hundreds. The undesirable by-product, of what is basically a change from potential energy to kinetic energy and finally to heat, is cavitation for liquids and high aerodynamic noise for gases. While cavitation can be very destructive for metals touched by the fluids, it also produces highly annoying sound.
Current efforts to combat such undesirable effects involve drilled cages installed in globe style valves which, while reasonably effective, they are rather small and their fluid impedance stays relatively constant with travel of the valve.
Other attempts are made to place multi-hole resistance plates downstream of a throttling valve. Their use is limited to fairly constant flow rates. For example, if the pressure drop across the valve is 16% of the total head loss at 100% of the flow rate, and that of the plate being 84%; then when the flow is cut in half, the pressure drop across is reduced to 0.5 squared×84=21%. Subsequently, the drop across the valve is now a very high 79%.
Sliding gate valves are reasonably effective in reducing noise and cavitation for moderate pressure reduction. The problem with this type of valve is, they exhibit very high friction forces due to the fact that one of the sliding plates is forced by the fluid pressure against a static second plate.
The current invention overcomes such friction forces, since instead of flat plates, the multi-passages sliding elements are a pair of concentric tubes. There is basically no friction, since the pressure acts on each portion of the inside circumference and thereby cancels out.
In contrast to other devices having constant area flow passages, this invention can vary the exposure of each flow passage in form of a variation in the overlap of two circular holes with change in the placement of one of the tubes. This not only increases the fluid impedance at reduced flow rates, but also results in higher frequency of sound with throttling of gases. Higher frequencies are better absorbed by a pipe wall, hence there is less audible sound exterior of the pipe.
Finally, a highly desirable flow characteristic is achieved by the system of overlapping circular holes, since the area exposed by the overlap varies roughly to the square of the distance between the center of the two holes.
These and more of the advantages and features of the invention will become more clear in view of the following detailed description.